Although the invention applies to the fabrication of virtually any type of semiconductor device wafer, an example of fabricating a light emitting diode (LED) wafer is presented.
Gallium nitride based LEDs, such as for generating blue light, are manufactured by epitaxially growing semiconductor layers over a growth substrate (a wafer), such as sapphire. An active layer between p-type and n-type layers emits light having a peak wavelength, and the peak wavelength is determined by the material composition of the active layer. Such semiconductor layers may be on the order of a few tens of microns thick and very brittle. Metallization and other well-known processes are then performed on the LED wafer to, for example, remove the growth substrate, thin the LED layers, and form electrodes. The LED wafer is subsequently diced to form LED chips for packaging.
In cases where the growth substrate is to be removed from the LED wafer, such as to perform processes on the LED layers facing the growth substrate, or in cases where the growth substrate is to be thinned, such as for scribe and break singulation, a carrier wafer must be first bonded to the opposite surface of the LED wafer to provide mechanical support of the thin LED layers during removal or thinning of the growth substrate. After the carrier wafer is bonded to the LED wafer and the growth substrate is removed or thinned, any exposed LED layers may be further processed, such as thinning the LED layers and depositing thin films over the exposed LED layers. The carrier wafer may be connected temporarily or permanently. If the carrier wafer is temporary, methods have to be performed to de-bond the carrier wafer from the LED wafer. Silicon carrier wafers are commonly used due to their low cost and well-known characteristics. A silicon carrier wafer absorbs light from the LED and should be ultimately removed from the LED layers.
Using a carrier wafer (typically silicon) for mechanical support is commonly used in integrated circuit processes, especially where IC wafers are stacked to form three-dimensional (3-D) modules and conductive vias extend vertically though the IC wafers. Typically, the carrier wafer is bonded to the IC wafer using an intermediate-temperature (e.g., up to 250° C.) polymer adhesive so that the adhesive does not release at the expected IC wafer process temperatures. After the IC wafer processing is completed, de-bonding of the carrier wafer is performed at a high heat and with special tools and techniques (e.g., using Brewer Science's HT1010™ thermal-sliding de-bond process).
It is also known to apply a special light-to-heat conversion (LTHC) layer over a carrier wafer, then bond the carrier wafer to the device wafer using a special adhesive that melts at a relatively low temperature. A laser beam is then used to heat up the LTHC layer, which, in turn, melts the adhesive for de-bonding the carrier wafer. The LTHC material, special adhesive materials (e.g., 3M's LC3200 and LC5200), and special tools are available from 3M Company. The process is relatively expensive and time-consuming. Since the adhesive needs to melt at a relatively low temperature (the 3M adhesive is rated only up to 180° C.), the de-bonding process is not suitable when quality thin films need to be deposited, since such thin films typically need to be deposited at over 250° C.
In some processes for forming high quality thin films, such as a PECVD process, the process temperature is substantially greater than 250° C. The polymers used for temporarily bonding the carrier wafer to the IC or LED wafer must therefore be very high temperature polymers (e.g., >350° C.), requiring more complex and higher temperature de-bonding processes and tools.
What is needed is an improved technique for de-bonding a carrier wafer from a device wafer that is simpler than the prior art processes. It is preferable that the de-bonding process works with commonly used bonding layer materials, including high temperature polymers (e.g., BCB, PBO, polyimides, etc.), intermediate temperature polymers/adhesives (e.g., Brewer Science's HT1010™, 3M's LC3200/LC5200, etc.), low temperature glues/adhesives, and even metal (e.g., eutectic alloy) bonding materials.